Soldering of electronic circuits involves physical contact between conductive tip of soldering iron and electronic components, often with low threshold of damage by excessive electric currents causing condition called electrical overstress (“EOS”). It is desirable to provide a device and method that deals with reduction of specific type of EOS caused by transient currents with high-frequency spectral content. Such industry regulatory documents as IPC-A-610 and IPC-TM-650-2.5.33.2 provide limits to the magnitude of such transient signals.
These transient signals are caused by noise on power lines and by the soldering iron itself. The latter is becoming a lesser issue with time as the quality of soldering irons, especially of professional grade, improves. The electrical noise on power lines and ground in manufacturing facility, however, is not decreasing, largely due to increasing number of electrical and electronic equipment in manufacturing environment where each such device generates noise on power lines and ground. A typical soldering setup is depicted in FIG. 1. Soldering iron 100 is connected by cable 105 to the soldering station 110. Said station in turn is plugged via cable 115 and electrical plug 120 into a power outlet 125. Said power outlet is grounded to a facility electrical ground 130. Noise on power lines 132 shown in a screenshot of an oscilloscope (not shown) creates corresponding voltage on the tip 134 of said soldering iron. The circuit board 135 being assembled is also connected to ground 137 which may or may not be the same as said ground 130. In presence of noise on power lines and ground there may be a current 140 between the tip of said soldering iron and said circuit board shown as screenshot 145. This current can cause damage to the electronic components as described in “EOS from Soldering Irons Connected to Faulty 120 VAC Receptacles,” Raytheon, W. Farrel et. al. ESD Symposium Proceeds, 2005.
FIG. 2 shows slightly modified arrangement also common in electronic assembly environment. As seen, said circuit board is not grounded directly, but is placed on a static-dissipative mat 146 which in turn is connected to said ground 137 often via the resistor 147. Even though this arrangement may prevent high current from the iron as long as this current is either DC or of very low frequency, such as that of mains (50/60 Hz), the inevitable parasitic capacitance 148 between said board 135 and said ground 137 makes impedance for high-frequency transient signals from the tip of the iron very low which allows for high currents.
The nature of such electrical overstress becomes evident in analysis of FIG. 3. In FIG. 3 said plug 120 of said soldering iron station 110 connects to said electrical outlet 125. Said outlet is, in turn, connected to the mains power lines live (L) 150, neutral (N) 155 and said ground (G) 130. Mains typically have high-frequency noise comprised mostly of transient signals shown here as differential voltage (between live and neutral) 170 and common mode noise (between live/neutral and ground) 175 and 180 accordingly. Said mains lines have inherent distributed impedance comprised of inductance 185, resistance 190, and distributed capacitance 195 between all the lines. Similar distributed impedance is present to some extend in all wires shown in FIG. 2. Said soldering station 110 is typically comprised of AC to DC converter 200 that lowers the mains voltage to typically 12V or 24V and then converts this AC voltage to DC. Said soldering station also typically has temperature control mechanism 205. Said soldering station is connected via said cable 105 to a heating element 210 enclosed in a ceramic element 215 which, in turn, is thermally connected to said soldering tip 134. The soldering station 110 by itself can be a generator of high-frequency transient signals 220, though the noise from power line and ground is typically substantially higher. Parasitic capacitance 222 between input and output of said solder station 110 serves as conduit of noise between the power line and output. Parasitic capacitance 223 between the heating element and the solder tip serves as conduit for noise to said tip. The description below addresses only the arrangement of FIG. 1 for simplicity and brevity of this disclosure. It should be noted that the arrangement of FIG. 2 behaves substantially the same.
Said circuit board 135 to which the soldering iron solders components to is grounded for the purposes of protection against electrostatic discharge (ESD). The grounding wire 225 possesses said distributed resistance 195 and inductance 200 similar to other cables. Said grounding wire is eventually connected to the facility ground 130. Factory ground usually contains high-frequency noise shown in this figure as a noise source 230 caused by a number of factors, including leakage current from live and neutral wires and inductive and capacitive coupling with the said wires.
Difference in voltage between said tip 134 and said circuit board 135 causes high-frequency current 235 with the waveform shown in the screenshot 240. This current can be quite high. FIG. 4 shows the results of measurements. FIG. 4a depicts the test setup where the parts of it are similar to those of FIG. 1. A current sensor 230 with the wire 232 that is used to measure current is connected between said tip 134 of said iron 100 and the grounded circuit board 135. Said current probe is connected via cable 234 to a high-speed digital storage oscilloscope 236. Another channel of said oscilloscope is connected via cable 238 to a high-voltage adapter 240 plugged into the said mains outlet 125. Said adapter provides safe means for said oscilloscope to measure high-frequency signals on high-voltage power lines.
The setup in FIG. 4a allows for simultaneous measurements of both high-frequency signals on the mains and the current between the tip of soldering iron and the circuit board. FIG. 4b depicts a screenshot of said oscilloscope. As seen, the transient signal on the mains (Channel 2) is perfectly synchronized with the current spike shown on Channel 1. Said current probe 220 has conversion factor of 5 mV/mA, meaning that 5 mV of signal on the screen corresponds to 1 mA of current. As shown, the peak current is 18.8 mA which is very high for sensitive components.
FIG. 5a depicts an attempt to mitigate high current problem by including a typical power line EMI (electromagnetic interference) filter 250 connected between said power lines and said soldering station. The schematic of the filter shown is for example only—it shows that the filter has both common-mode and differential-mode filtering, i.e. typically complete set of filters for reduction of noise on power lines. Ground of said circuit board is connected to ground in the same way as in FIGS. 1, 2, 3 and 4. FIG. 5b depicts the results. The measurement setup is the same as in FIG. 4a. Similarly to FIG. 4b, there is current 235 between the tip of the soldering iron that is synchronized to the noise on the mains. The signal is quite different, though. While it is lower in maximum value, the waveform now contains frequencies not present before. This correlates with the investigation on the subject (EOS from Soldering Irons Connected to Faulty 120 VAC Receptacles, W. Farwell et al., Raytheon, ESDA Symposium 2005). As seen in this paper, simply inserting a conventional EMI filter between the power line and the soldering station does not present a satisfactory solution of significantly reducing current between the tip of the iron and the circuit board.
It is desirable to have a practical solution that greatly reduces high-frequency current between the tip of the soldering iron or other tools and sensitive components on the circuit board, thus protecting these components from electrical overstress. It is also desirable and often required by various regulations, including safety regulations, to have low resistance to ground for DC and 50/60 Hz signals as specified by numerous standards and regulations, including but not limited to: IPC-TM-650, MIL-STD-2000, ANSI ESD DS 13.1 and others.